1. Field of the Invention
The present invention relates to a voltage detecting circuit for detecting the voltage of a battery or other similar device and outputting a signal that indicates whether the voltage is high or low with respect to a predetermined voltage, and to a battery device suitable for battery using equipment that is able to fully utilize the potential of the battery.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. H11-258280 discloses one example of a known conventional voltage detecting circuit and battery device using this circuit. FIG. 4 of this example shows a voltage detecting circuit 102 and a battery device 101 using this circuit. A battery device 101 includes a voltage detecting circuit 102; series-connected resistors 104, 105, having one end grounded and for generating an input voltage VBAT of the voltage detecting circuit 102; and a battery 103 connected to the other end of the series-connected resistors 104, 105 that serves as the power source of a battery using equipment. The voltage of the battery 103 is divided by the series-connected resistors 104, 105, and this divided voltage is input into the voltage detecting circuit 102 as an input voltage VBAT. The voltage detecting circuit 102 compares the input voltage VBAT with a detection threshold value voltage VTH. When the input voltage VBAT is higher than the detection threshold value voltage VTH, the voltage detecting circuit 102 determines that there is a battery residual quantity available and outputs a high-level voltage as a detected result to an output terminal OUT. Conversely, when the input voltage VBAT is lower than the detection threshold value voltage VTH, the voltage detecting circuit 102 determines that there is no battery residual quantity available and outputs a low-level voltage as a detected result to the output terminal OUT. The output terminal OUT is connected to other electronic components (not shown in the figure) which are included in the battery using equipment.
The voltage detecting circuit 102 includes a threshold value voltage generator 112 that selectively generates different threshold value voltages (high-side and low-side) depending on whether the input voltage VBAT increases (changes from low voltage to high voltage) or whether the input voltage VBAT drops (changes from high voltage to low voltage), a comparator 111 that compares this threshold value voltage (detection threshold value voltage VTH) with the input voltage VBAT, an inverter 124 that inverts the output of the comparator 111, an output switching element 113 that receives the output of the inverter 124 and outputs the voltage detection result from the output terminal OUT, and a reference voltage generator (REF) 114 that generates a reference voltage VREF. The output of the inverter 124 is also used for selecting one of the two threshold value voltages in the threshold value voltage generator 112. The reference voltage VREF is used as a reference for these threshold value voltages.
The threshold value voltage generator 112 and comparator 111 achieve hysteresis of the input voltage detection. By virtue of this achievement, when the input voltage VBAT approximates the detection threshold value voltage VTH, the repeated turning ON and OFF o the output switching element 113 caused by minute levels of noise resulting in instability of the output of the output terminal OUT is prevented.
The battery device 101 selects the high-side threshold value voltage when the input voltage VBAT increases and selects the low-side threshold value voltage when the input voltage VBAT drops. Accordingly, when the battery 103 is removed, as the input voltage VBAT drops, the low-side threshold value voltage is set as the detection threshold value voltage VTH. When the battery 103 is then reinserted, as the input voltage VBAT increases, the high-side threshold value voltage is set.
FIG. 5 shows the relationship between the input voltage VBAT and the detection threshold value voltage VTH. The figure shows a case in which there is only a little residual battery quantity available, that is, a case in which the battery voltage has gradually dropped from a state in which the input voltage VBAT exceeds a high-side threshold value voltage (for example, 2.9V) in the initial usage period of the battery 103 to a state in which the input voltage VBAT is a voltage (for example 2.6V) between the high-side threshold value voltage and a low-side threshold value voltage (for example 2.5V). When the battery 103 is removed and the input voltage VBAT drops and intersects the low-side threshold value voltage, the output terminal OUT switches from the high level to the low level and the detection threshold value voltage VTH is set to the high-side threshold value voltage. When the battery 103 is then reinserted subsequent to the input voltage VBAT having reached the ground potential (0V), the input voltage VBAT increases and exceeds the low-side threshold value voltage value. However, it does not reach and does not intersect with the high-side threshold value voltage and, as a result, the output terminal OUT remains at the low level. Accordingly, while the battery using equipment operates as if residual battery quantity is available until the battery 103 is removed, when the battery 103 is temporarily removed and then reinserted, a phenomenon is established in which there is a state of no residual quantity of the battery 103 available, which prevents the battery using equipment from starting up. This precludes the battery using equipment from being able to be used to the limit of the available residual quantity of the battery 103 and reduces the usable life of the battery.